As long as you're not ragging it everywhere, a good but rough indicator is 10 miles per litre.
98-99 bikes have an 18 litre tank, so riding sensibly you should expect 180 miles.
00-01 bikes have a 20 litre tank.
02-03 bikes have a 22 litre tank.
There is no exact mathematic answer simply because everyone rides differently, some bikes will be more efficient than others, and circumstances will be different per person.
For instance I spend a lot of time in daily traffic, so I've never even bothered checking mpg, whereas a steady motorway rider will get full benefits.